Blue Dragon hands down, yes it's lawful evil but the lawful part works into your advantage. It ensures the mercenaries will honor their deals, helping who pays without having to worry about them turning on the party that hired them. Add to that, Blue Dragons covet valuable and talented creatures as described in the MM, a mercenary band would make for a fine method to find such individuals and have them rise in the ranks.
In the same entry, it mentions how Blue Dragons are extremely protective of their lair and keep it secret to even most of their trusted inner circle, allowing the reveal of a dragon leader to be a surprise for even the higher ranked npc's in the group. In addition to the free 'recruitment' and 'filtering' of weak/strong creatures the mercenary group provides the dragon, it also gets a constant source of gold that only increases the more talented/strong the group becomes.
Finaly, the pride and vanity of the Blue Dragon could explain why the group only accepts those with some connection to dragons, seeing other creatures as too inferior to be worth its attention
I do suggest either straight up giving the dragon the shapeshift ability of the metalic dragons or using the variant rule for dragons as innate spellcasters so it can use the shapeshift spell to walk around without revealing its true nature.